Migrate from Other Launchers

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From the Vanilla Minecraft Launcher

The official Minecraft Launcher doesn't have an export function — everything lives inside a single .minecraft folder. Migration is entirely manual, but straightforward once you know where things are.

Step 1: Locate Your .minecraft Directory

If you've used a custom directory, you can find it in the official launcher under Installations → (…) → Open Folder.

Step 2: Create a Matching Instance in Polymerium

1. Click the + button in Polymerium.
2. Choose New Instance.
3. Select the same Minecraft version you were playing.
4. If you use mods, select the same mod loader (Fabric, Forge, NeoForge, or Quilt) and a compatible version.

Not sure which loader or version? Open .minecraft/versions/ — if you see a folder like 1.21.4-fabric-0.16.9, you were using Fabric 0.16.9 on 1.21.4. If all folders are plain version numbers like 1.21.4, you were playing vanilla (no loader).

Step 3: Add Your Mods

The official launcher stores mods directly in .minecraft/mods/. You need to add them through Polymerium instead:

1. Open .minecraft/mods/ in a file explorer.
2. For each .jar file, search for the mod by name in Polymerium's Add Package dialog (it searches CurseForge and Modrinth). This way the mod is tracked and can be updated.
3. If a mod isn't available on CurseForge or Modrinth, you can place the .jar directly into the instance's import/mods/ directory — but it won't be managed or updated automatically.

Step 4: Transfer Your Worlds and Personal Data

This is the most important step. Your saves, resource packs, screenshots, and settings live in various subfolders of .minecraft/. You need to copy them into your Polymerium instance's persist/ directory.

Which folders to copy:

Important: For each directory you copy (like saves/), create a .keep file inside it (e.g., persist/saves/.keep). This tells Polymerium to link that directory into the game on deploy. Files placed directly in persist/ (like options.txt) are linked automatically. See Four-Layer Directories → for details.

Step 5: Deploy and Play

1. Click Deploy (or just Launch — Polymerium deploys automatically).
2. Your worlds, settings, and resource packs will appear in the game.
3. Verify everything looks correct — check your worlds are there, settings are right, etc.

Troubleshooting

• Worlds are missing: Make sure persist/saves/.keep exists and you deployed after copying.
• Settings reset: Check that persist/options.txt is present — this file controls in-game settings like render distance and keybinds.
• Mods aren't loading: Verify the mod loader version matches. A Fabric mod won't load on Forge, and vice versa.

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From Prism Launcher or MultiMC

Polymerium can directly import MultiMC-format modpacks, which Prism Launcher also uses.

Step 1: Export from Prism

1. Right-click the instance in Prism Launcher.
2. Select Export Instance.
3. Choose MultiMC format (or Modrinth / CurseForge format — Polymerium supports all three).
4. Save the .zip file.

Step 2: Import into Polymerium

1. Click the + button in Polymerium.
2. Choose Import from file.
3. Select the .zip you exported from Prism.
4. Polymerium auto-detects the format, resolves mods, and creates the instance.

Step 3: Set Up Your Account

If you were using a Microsoft account in Prism:

1. Go to Accounts in Polymerium.
2. Add your Microsoft account via Device Code Flow.
3. Set it as the default or bind it to the imported instance.

What Transfers

Manual File Transfer (Saves, Screenshots)

If you want to keep your saves and screenshots:

1. Find Prism's instance directory (usually ~/.local/share/PrismLauncher/instances/).
2. Copy saves/, screenshots/, resourcepacks/, and options.txt from the Prism instance.
3. Paste them into your Polymerium instance's persist/ directory. For directories like saves/, create a .keep file inside (persist/saves/.keep) so they get linked into the game. See Four-Layer Directories → for details.
4. Deploy — the files will be projected into the game directory.

Things to Know

• Polymerium uses symlinks — make sure Developer Mode is enabled on Windows.
• Mod files are shared — you won't need to re-download mods that are already in CurseForge/Modrinth format. Polymerium resolves them from its cache.
• Your old launcher instances are untouched — importing creates new instances; nothing is deleted or modified in your previous launcher.